C library: make sleep() branch free

[tautschnig]

The if(...) assignment can be replaced by an assume statement, which
avoids a branch.

Also, enable the regression test of the sleep function.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #6095 (50d64cd) into develop (823e1b2) will increase coverage by 0.07%.
The diff coverage is 96.96%.

@@             Coverage Diff             @@
##           develop    #6095      +/-   ##
===========================================
+ Coverage    74.45%   74.53%   +0.07%     
===========================================
  Files         1447     1447              
  Lines       157746   157761      +15     
===========================================
+ Hits        117453   117584     +131     
+ Misses       40293    40177     -116     

Impacted Files	Coverage Δ
jbmc/src/janalyzer/janalyzer_main.cpp	100.00% <ø> (ø)
jbmc/src/java_bytecode/assignments_from_json.cpp	97.26% <ø> (ø)
jbmc/src/java_bytecode/ci_lazy_methods.cpp	99.47% <ø> (ø)
jbmc/src/java_bytecode/ci_lazy_methods.h	100.00% <ø> (ø)
jbmc/src/java_bytecode/ci_lazy_methods_needed.cpp	98.24% <ø> (ø)
jbmc/src/java_bytecode/ci_lazy_methods_needed.h	100.00% <ø> (ø)
jbmc/src/java_bytecode/convert_java_nondet.cpp	81.15% <ø> (ø)
...java_bytecode/create_array_with_type_intrinsic.cpp	97.67% <ø> (ø)
jbmc/src/java_bytecode/expr2java.cpp	86.61% <ø> (ø)
jbmc/src/java_bytecode/expr2java.h	100.00% <ø> (ø)
... and 258 more

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[NlightNFotis]

LGTM